Regulations throughout the world require regular survey of gas networks to check for unknown leakage. The reason for this is primarily to protect the public from fires and explosions resulting from gas collecting in buildings and other confined spaces. The leak survey is typically carried out by moving a detector, sensitive to one of the main components of the gas, over the surface of the ground above the pipe carrying the gas.
For compressed natural gas (CNG) the main component is typically methane while Liquefied Petroleum Gas (LPG) contains mainly Butane and Propane. In addition to these two more common gas types there is a growing production of biogas even though the pipe network is still very limited. Bio-gas contains mainly methane. All such flammable gases distributed in pipelines or vessels are denoted utility gas in the following.
A fourth type of flammable gas used in some parts of the world is called coal gas or manufactured gas (MG). This contains high levels of hydrogen and is excluded from above the definition of utility gas.
If a utility gas signal is registered it can originate from two main sources; the gas installation under inspection or from biological decay processes. Examples of such biological sources are covered waste dumps, decomposing sewage, landfills etc. Gas originating from such sources is known by numerous names including marsh gas, soil gas, landfill gas and sewer gas. In the following the term swamp gas is used as a common denominator for flammable gas sources of biological origin. In order to avoid unnecessary excavations on swamp gas signals it is desirable to be able to distinguish between real gas leaks from the pipe under survey and indications from swamp gas.
The most common method used to discriminating between utility gas and swamp gas is making an analysis of the components of the gas to determine the presence of ethane gas and/or heavier hydrocarbons such as propane and butane. Most natural gas sources contain 0.5-8% of ethane while swamp gas does not contain any significant amounts of ethane.
There are at present no commercially available gas sensors with a high enough selectivity between methane and ethane to make such an analysis possible in real time. The analysis is, therefore, typically carried out using a gas chromatograph (GC) separating the different components of the gas sample into pulses exiting the chromatograph column at different times. Due to this time separation of the different components, the analysis is possible even with a non-selective sensor.
The GC can be a lab instrument in which case a gas sample is collected in some type of container that is sent off to a laboratory for analysis. Field operable GCs are also available. These can be dedicated instruments or small GC modules integrated into a pipeline leak surveying instrument. The field GCs and especially those integrated into a leak detector are generally less sensitive than the lab units and sample concentration must be above 1% or at least 0.5% to make a certain detection of ethane in the sample.
This fact often makes it necessary to drill probe holes through the paving to allow a high enough concentration of gas to be collected. This makes the effective test time considerably longer and typically in the order of 30 minutes or more.
Another way of discrimination is to detect the presence odorant either by smelling the gas or by using a specific detector. The “manual” method is simple and low cost but not entirely reliable and detectors for the odorants are typically higher grade GCs associated with similar delay and higher costs than for those used for ethane detection. A better performing GC is needed as the odorant concentration is several orders of magnitude lower than the ethane concentration.
In any case, certain discrimination between swamp gas and utility gas is a slow and not always entirely reliable process and there is a need for a simpler and faster method.